Soil respiration and heterotrophic respiration in a cool-temperate forest with sika deer-induced alteration of understory vegetation

Abstract

Overpopulated ungulates reduce the biomass of understory vegetation and promote the expansion of unpalatable plants in world forests. Such understory degradation may influence soil respiration (R_s) and heterotrophic respiration (R_h). Here, we examined this possibility in one cool-temperate forest in southern Kyushu, Japan. At the study site, the dominant understory vegetation, dwarf bamboo (Sasa; Sasamorpha borealis), has been patchy lost and replaced by an unpalatable shrub, Asebi (Pieris japonica), owing to sika deer feeding. We targeted three understory vegetation types: Sasa understory (SU), no understory (NU), and Asebi understory (AU). The R_s, R_h, soil temperature, and soil volumetric water content (SVWC) were measured at three points in each understory type using an automatic opening/closing chamber system from August 2022 to November 2023. We then evaluated biotic and abiotic factors, including surface litter amount, fine root biomass, and soil physiochemical properties, to examine their effects on the temperature sensitivity proxy (Q₁₀) of R_s and R_h. Annual R_s and R_h were estimated using continuously measured soil temperature data and temperature-R_s and R_h relationships. The temporal variation of R_s and R_h was strongly affected by soil temperature and weakly by SVWC for all understory types. The spatial variation in Q₁₀ of R_s and R_h was explained by fine root biomass and surface litter amount, respectively, regardless of understory type. Differences in annual R_s and R_h among understory types were comparable to those among measurement points in the same understory type. This was due to the similar soil temperature and Q₁₀ of R_s and R_h among understory types. Thus, inner-site R_s and R_h variations could be generated by the spatial variations in soil biotic factors, regardless of understory vegetation type in our study site. This means our study cannot clearly verify whether overbrowsing increases R_s and R_h, highlighting the necessity for future research.